1. Prior Developments
In many applications involving circuits with several generating and/or receiving systems of an electrical nature, the form of the energy being generated or absorbed must be adapted by each system in terms of predetermined imperatives intrinsic to the systems concerned. Illustratively a set of batteries integrated into an emergency unit when acting as a load must be charged according to a specific rating, and in case of power main failure must operate as a generator and deliver power adapted to the needs and compatible with the characteristics of the battery.
The most widespread solution for controlling energy exchanges between several systems comprises inserting energy converters between the pair of systems exchanging the energy and controlling each converter in relation to the energy exchange which is desired between two systems involved by conventional power control methods (pulse-width modulation, rectification phase control and the like). Each converter inserted between two particular electrical systems is selected to have at its input the features of one system (frequency, voltage. . .) and at its output the features of the other.
In almost all cases, the systems being considered are DC or low-frequency (industrial frequencies: 50 Hz or 400 Hz in aeronautics), and often it is necessary to introduce voltage isolation and to match the impedances between the systems. In the low-frequency case, the required transformers, filters etc. are heavy, bulky and costly; in the case of DC, additional AC stages must be provided, which either are low-frequency and incur the cited drawbacks, or preferably high-frequency stages are used. This latter type of stages (high-frequency) sometimes are used in low-frequency applications to elimimate the above cited drawbacks, and this is presently also the trend.
However under those conditions the conventional solutions for controlling converters are applicable only with difficulty because of their technical limitations and it becomes necessary to resort to other methods. In the case of an exchange between two systems, one of these methods comprises adjusting the power between the two systems by inserting between them an oscillation circuit and by varying the oscillating circuit and by varying the oscillation frequency of that circuit. Illustratively the object of the solution below is to control the exchanges between two DC or lowfrequency AC sources and is described in many publications, in particular in the following:
(1) F. C. Schwarz, "A Method Of Resonant Current Pulse Modulation For Power Converters"; IEEE Transactions on Industrial Electronics and Control Instrumentation, Vol. 17, No. 3, May 1970;
(2) F. C. Schwarz, "An Improved Method of Resonant Current Pulse Modulation For Power Converters", IEEE Transactions on Industrial Elecronics and Control Instrumentation, IECI-23, No. 2, 1976
(3) F. C. Schwarz and J. B. Klaassens, "A Controllable Secondary Multikilowatt DC Current Source With Constant Maximum Power Factor In Its Three-phase Supply Line"; IEEE Transactions on Industrial Electronics and Control Instrumentation, Vol. 23, No. 2, 1976;
(4) F. C. Schwarz and J. B. Klassens, "A Controllable 45 KW Current Source For DC Machines"; IEEE Transactions on Industry Applications, Vol. 1A-15, No. 4, July/August 1979;
(5) F. C. Schwarz, "A Double-sided Cycloconverter"; Proceedings of the 10th IEEE Power Electronics Specialists Conference, San Diego, June 1979;
(6) F. C. Schwarz and Moize de Chateleux, W.L.F.H.A., "A Multikilowatt Polyphase AC/DC Converter With Reversible Power Flow and Without Passive Low Frequency Filters"; The 10th IEEE Power Electronics Specialists Conference, San Diego, June 1979;
(7) F. C. Schwarz and J. B. Klaassens, "A Reversible Smooth Current Source With Momentary Internal Response For Nondissipative Control of Multikilowatt DC Machines"; IEEE Summer Meeting, Minneapolis, Minnesota, July 13-18, 1980.
This solution comprises assigning to each low frequency or DC source one converter and to interconnect the two converters by a series LC oscillation circuit. One of the converters applies a variable frequency and the power exchanged between the two sources is adjusted by varying this frequency; the other converter control solely determines the direction of the exchange of energy.
The main drawback of this type of device is in its very principle of operation: the frequency variation allowing to control the power must be possible over a large range in order that there may be practically significant regulation At the frequencies involved, such variations may make filtering difficult due to the width of the spectrum and therefore may become sources of environmental pollution (danger of interfering with other electrical equipment, even if remote, electromagnetic interference), on one hand; on the other hand, some power regulation--especially near zero power--sometimes is difficult to implement with such devices (because requiring very high or very low frequencies in relation to the average operational frequency).
Furthermore, this principle of control by varying the frequency is applied to an exchange between two electrical systems, and appears difficult to transpose to an exchange between a larger number of electrical systems. 2. Objects
The primary object of the present invention is to create a new device for controlling energy exchanges which benefits from the advantages of high frequencies while being free fron the above described drawbacks.
One particular object is to permit operating at a constant high frequency in order to achieve a sharply defined spectrum and accordingly to make filtering much easier.
Another object of the invention is related to this constant, high-frequency operation and is to achieve very short response times independent of the power regulation.
A further object of the invention is to achieve very wide ranges of power variation wthout any particular difficulty near zero power.
Still another object of the invention is to considerably reduce the weight, the bulk and the cost of the accessory passive components required for each application, such as filters, isolation transformers or impedance-matching transformers.
Still a further object of the invention is to improve the converter efficiency by operating the converter, regardless of type, in much more advantageous modes than in the prior instances of frequency variation.
Yet another object of the invention is to create a device with very wide applicability which, in particular, is capable of controlling energy exchanges between an arbitrary number, equal to or exceeding 2, of systems.